1. Field of the Invention
This invention relates generally to electrostatic coalescers, and, more particularly, to an improved and compact coalescer, in which a turbulent flow of emulsion is passed through narrow flow channels and subjected to a high intensity field for more effective coalescing action.
2. Description of Related Art
The use of electrostatics to enhance the coalescence of water droplets in an oil continuous emulsion has been used by the oil industry for many years. Electrostatic coalescers are commonly used at oil producing facilities to remove unwanted water from the crude oil, and are also used at refinery facilities to remove salt (dissolved in water) from crude feed stock. Commercial electrostatic dehydration equipment is available from a number of reputable manufacturers, including Kvaerner Process Systems of Calgary, Canada, the assignee of the present invention, in different parts of the world. This equipment may use proprietary designs, but they are all based on the same principles. Namely, an emulsion containing a dispersed water phase is introduced into a zone subjected to an electrostatic field. The electrostatic field causes the dispersed water droplets to coalesce by polarizing adjacent drops and causing a dielectrophoretic force of attraction between them. After small water droplets have coalesced into larger droplets, they are separated by gravity. Traditionally the emulsion is treated in a large vessel so that the fluid velocity is very small, allowing the water droplets to settle to the bottom of the vessel where the water can be removed. The electrodes used to impose the electrostatic field inside the vessel are normally quite large and spaced in a manner to minimize disruptions to the fluid flow patterns that could cause mixing of the water droplets in the oil phase.
Applied voltages to create the electrostatic fields in typical commercial units range from 10,000 to 30,000 volts (rms voltage given throughout document). Operating voltage settings of 15,000 to 20,000 volts are common. This requires carefully designed and expensive insulators, electrode hangers and entrance bushings. The high voltage bushings are subject to failure due to the severe service and difficult operating environment with temperatures reaching up to 150.degree. C. (300.degree. F.). This requires periodic shutdowns for replacement of worn or failed bushings.
Depending on the process conditions and the design of the electrostatic coalescer, problems can occur when emulsions with high water concentrations enter an electrostatic field. Many commercial electrostatic coalescers utilize bare metal electrodes or grids. High water concentrations greatly increases the conductivity of the emulsions, and with the high voltages applied to the field, large current draws can occur and even shorts to ground. This can cause both process and equipment failures. To avoid this situation, the traditional approach has been to locate the energized electrodes well away from the grounded electrodes or any other grounded part of the system, or to pretreat the emulsion to remove much of the water. Traditional spacing inside commercially available electrostatic coalescers is about 9 to 24 inches between electrodes and ground. This large spacing between electrodes causes a less intense electrostatic field to be generated, and is, thus, less effective in coalescing water droplets. Dehydration performance deteriorates.
Recent laboratory research work carried out by Statoil of Norway and the University of Southampton in England, has demonstrated the destabilization of water-in-oil emulsions using electrostatics under turbulent flow conditions. This work has led to the development of new electrode configurations that can be used in a wide variety of commercial electrostatic coalescers. The new configurations enable small water droplets to coalesce into larger droplets while flowing turbulently. Turbulent flow provides good mixing which increases the probability of water drops coming into close proximity with other water drops. This is achieved by creating high flow velocity within narrow flow channels between electrodes. Lower voltage is used with this new electrode configuration, however, and because the electrodes are more closely spaced, higher field intensities than in conventional units are actually observed. This higher field intensity, coupled with the turbulent flow between the electrodes, results in more effective and improved coalescing action. However, there is a limit to which field intensity and turbulent mixing will be effective because water drops can break up under the action of an extremely intense field and turbulence induced stresses.
The energized electrodes are insulated with a layer of material with high dielectric strength. This prevents high current flow and short circuits when a water-in-oil emulsion with high water concentration enters the field, thus eliminating the need and cost of pretreatment of the emulsion. The new electrode configurations can handle water cuts in excess of 40%, up to the point where the emulsion inverts and the water become the continuous phase.